Despite nearly two decades of evolution, many commercially-available Li-ion cells continue to have poor materials utilization, with only about 50% of cell volume devoted to active materials even in cells designed for high energy density (e.g., state-of-art 18650s). Electrode thickness or lack thereof, is believed to be a cause of the low volume and mass efficiency. Electrodes that can meet the duty cycles of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) are typically only 60-100 μm thick; techno-economic modeling has shown that by simply doubling electrode thickness, the cost of a PHEV pack could be reduced by about $600. However, making electrodes thicker generally leads to increased diffusion pathways of the charge carrying lithium ions through the liquid electrolyte filled pores. Hence, at high rates only a fraction of the stored energy can be delivered since the battery fails due to limited ion transport in the liquid phase.
Increasing the porosity of the electrodes will increase the ion transport in the electrolyte but simultaneously diminish the energy density of the battery. Instead, changing the tortuosity of the electrode has shown to enable high rate capability while maintaining the energy density. The tortuosity τ=ε·σ0/σ is a measure of the effectiveness of the pore structure in respect to mass transport, where ε is the porosity, σ0 is the transport coefficient of ions in the electrolyte and σ is a measured transport coefficient of ions in the structure. Consequently a straight pore, relative to the diffusion direction, is equal to tortuosity of unity, while more random pore shapes can reach much higher values.
Measured tortuosities in batteries can exhibit values up to 27 but are usually in the range of 2-5, showing that there is much room for improvement. Low tortuous structures can be obtained by anisotropic pores, which are oriented in the direction of ion diffusion. These structures have shown to greatly improve the delivered capacity at high C-rates. For many applications, the anisotropic pores need high aspect ratios and small diameters.
While the benefits of low tortuosity pores in electrodes has been recognized, commercial methods for incorporating such pores into electrodes has had limited success. Accordingly, improved methods of making such pores, in electrodes and other porous structures, would be desirable.